The present invention relates to illuminators used in industrial instrumentation in which it is desired to illuminate an object which is being inspected by a sensor or camera. An example is the illumination of objects on a production line which are being inspected by a machine vision system.
A typical machine vision system consists of an illuminator, a camera, and a control system for enabling the illuminator and camera in combination to capture an image of the object which is under inspection and for processing the image and initiating the desired action. The action may be marking the product as acceptable or rejected. The illuminator is an important part of the system, and properties of the illumination such as its brightness, its uniformity, its angle of incidence, its colour, and its degree of collimation can have a significant influence on the overall performance of the complete vision system. Additionally, the physical size of the illuminator is very important, as for some vision systems there is very little physical space available. For example, it may need to be located within a camera housing. Several types of illuminator have been used with machine vision systems, for example incandescent bulbs, fluorescent tubes, lasers, Xenon flash tubes, halogen bulbs combined with fibre light guides, and light emitting diodes (LEDs). Due to their relative cheapness, physically small size, long lifetime, fast switching speed and reasonable efficiency, LEDs have become increasingly popular.
FIG. A illustrates the main features of a typical prior art ring illuminator, such as for example displayed at the Volpi(trademark) website, or in the Electronic Imaging Catalog of Edmunds Industrial Optics Inc(trademark). The illuminator consists of an arrangement of light emitting diodes arranged either within a housing, or on a printed circuit board within a housing, with a hole in the centre of the housing and board through which a camera or sensor can view the objects which are being illuminated. Each LED is in its own package, usually lensed so as to provide a xe2x80x9cviewing anglexe2x80x9d of between 4 degrees and 120 degrees. By viewing angle is meant the full width angle measurement in the far field at the half power points of the light which is being emitted from the package. In general, the light from these packages has a cross-sectional profile which is such that it can not be described by a smooth mathematical function such as a Gaussian function, but has undesirable intensity variations caused by the structure of the LED chip and/or by the arrangement of the LED within the optical elements of the package. It can be seen additionally from FIG. A that there is the potential for a lot of structure (non-uniformity) in the illumination pattern which is produced when these packaged LEDs are used to make a ring illuminator depending upon how well the beams from the individual LEDs mix. This can be improved by adding diffuser material either within the individual LED packages or on the exterior of the housing of the illuminator, but the effect in both circumstances is to reduce the efficiency of the system. It can also be seen from FIG. A that the overall volume of the illuminator is very much determined by the size of the individual diode packages, which are typically several millimeters in diameter.
A problem with such illuminators is that the pointing accuracy of individual LEDs is poor, leading to lack of uniformity of the illuminated area. Also, the uniformity of the light produced by the individual LEDs is poor, and even after they mix on the target, the resulting uniformity is poor, perhaps xc2x125%.
Another disadvantage is that the overall brightness of the illumination is limited by the packing density of the individual packaged LEDs, which are typically housed in 5 mm or 3 mm diameter acrylic packages. For example, a 30 mm diameter ring of 5 mm LEDs can only hold about 16 of these lensed LEDs.
In order to produce a shorter working distance it has been proposed to arrange the LEDs at an angle to the optical axis. This means that the illuminator works well at a particular working distance, which is determined by the position at which the beams from the individual LEDs coincide. However, there is no facility for adjusting the working distance.
U.S. Pat. No. 5,822,053 describes a system for improving the uniformity and for matching the angle of illumination with the field of view. This consists in essence of bending the metal legs on the individual packaged LEDs to adjust the orientation of the beam from each LED. By observing the pattern on the target plane, it can then be ensured that the uniformity is improved to the extent that is possible, given the non-uniformity of the individual LEDs to start with. Following adjustment, the LED packages are fixed in place with epoxy. This technique can only improve uniformity up to a point, and does not address the other fundamental limitations of using a ring of lensed LEDs, such as limitations of brightness, compactness, and scaleability to small sizes.
U.S. Pat. No. 5,580,163, describes a mechanical adjusting system to enable the angle of incidence of the illumination from a ring illuminator consisting of the lensed LEDs to be varied. This is mechanically complex, and adds significantly to the manufacturing cost of the system and to its volume and mass. Also, it does not address uniformity and brightness problems.
Use of annular lenses has been described in the prior art. U.S. Pat. No. 4,567,551 describes a multidirectional surface illuminator which uses a Fresnel lens to redirect and focus the light from the light sources arrayed around the outside of an illuminator housing, so as to illuminate the object under inspection from a particular quadrant of the illuminator. It is very difficult to make a compact illuminator using this technique due to the arrangement of the light sources outside of the housing. Additionally, this arrangement does not achieve a desirable level of uniformity over a target area, and is not suitable for extension to very small sizes.
U.S. Pat. No. 5,690,417 describes a surface illuminator with a means for adjusting the orientation and inclination of the incident illumination, which includes light emitting diodes arranged in coaxial circles. Each coaxial circle of diodes may have their beam angles inclined at different angles to the angle of the axis of the housing. The diodes are energizable in pie-shaped sectors so that arcuate clusters of diodes may be illuminated to enable the orientation of the resultant illumination to be varied. The light emitting diodes are individually packaged so as to include a collimating lens and a Fresnel lens is used to focus and redirect the light towards the illuminated object. The illumination is similar to the prior art illuminator illustrated in FIG. A from the perspective of efficiency and uniformity. The only function of the Fresnel lens is to enable the variation of the angular inclination from the various segments, while using light emitting diodes mounted coaxially with the housing.
U.S. Pat. No. 5,897,195 describes an illuminator with a cylindrical or conical array of light emitting diodes producing collimated light beams that are redirected towards the illuminated object by a Fresnel-like diffuser. Again, this is a variation of the prior art illuminator shown in FIG. A which might have some advantages for producing particular angular inclinations. However, it is not very suitable for compactness, high uniformity or high efficiency.
These disadvantages with using discrete light sources such as LEDs have led to use of large illuminators for machine vision systems and for microscopy in order to achieve the required illumination intensity and distribution. These illuminators may, for example, comprise a ring-shaped fluorescent tube. These units are bulky and so impose design limitations on the instrument with which they are intended to be used, and they also are not efficient for producing monochromatic light. These illuminators may also comprise xe2x80x9ccold lightxe2x80x9d sources, consisting of a fan-cooled halogen bulb providing light via a light pipe and adapter, which are bulky and expensive.
Therefore, it is an object of the invention to provide a ring illuminator which has some or all of the following advantages:
is compact, providing a high ratio of brightness to illuminator volume,
may have a size within a large range extending down to a compact size, for example an illuminator having 10 LEDs with a housing diameter of less than 10 mm,
provides a uniform illumination intensity over the desired target area,
incorporates redundancy so that failure of some sources does not result in darker areas within the illuminated area,
has a relatively high optical efficiency,
can be readily focused to adjust the size of the illuminated area,
allows configuration for a required application at the design and manufacturing stages in a versatile manner.
According to the invention, there is provided an illuminator comprising a plurality of light sources mounted in a ring configuration, and an optical system for directing light from the light sources onto a target plane,
characterised in that,
the properties of the optical system and the spatial arrangement of the optical system with respect to the light sources is such that light emitted by the light sources mixes to uniformly illuminate the target plane.
Thus, the optical system properties and the spatial arrangement achieve uniformityxe2x80x94desirable for many industrial applicationsxe2x80x94in an efficient manner without the need for a large single light source such as a fluorescent bulb.
In one embodiment, the optical system comprises an annular lens. This is a very simple, compact, and effective optical system.
In one embodiment, the separation of the light sources and the lens is less than the lens focal length. This achieves a uniform illumination area which is greater than the area encompassed by the lens, and also achieves excellent spread in the radial direction.
In one embodiment, the lens comprises means for controlling spread in the radial direction sufficient for uniformity and adequate illumination intensity across the target plane.
In one embodiment, the effective diameter of the ring of light sources is greater than that of the lens by 2% to 10%. This helps to control the size of the illuminated area and to achieve excellent uniformity.
In one embodiment, the lens comprises means for allowing light from the light sources to spread primarily in the tangential direction so that any point on the target plane is illuminated by at least 25% of the light source. This is a very simple way of achieving uniformity and excellent brightness on the target plane.
In one embodiment, the light sources comprise means for emitting uncollimated light. This also contributes to spread of light, helping to achieve uniformity.
In one embodiment, each light source is a semiconductor die. This is a compact arrangement.
In one embodiment, the illuminator comprises a housing supporting the light sources and the optical system, said housing having a chamber for the light sources and a chamber for the optical system. This provides robustness and reliability.
In one embodiment, the light source chamber supports the light sources in efficient thermal contact with the housing. This contributes to reliability.
In one embodiment, the housing supports the optical system in a manner whereby it is accessible for adjustment by users. This allows excellent versatility.
The annular lens may have a Fresnel pattern, and preferably the Fresnel pattern comprising means for correcting for spherical aberration. The lens may alternatively comprise a diffraction pattern. The lens may have a convex shape on one side and be flat on the other side. The lens may comprise a plurality of individual lenses arranged in an annular configuration.
In one embodiment, the light sources are mounted on a circuit board. This both allows simple electronic control and provides mechanical support.
The circuit board may have electronic components to control or switch current to the light sources.
In one embodiment, the circuit board is flexible and is formed into a ring. This provides ease of manufacture.
In one embodiment, the drive circuits comprise means for providing different colour illumination. In another embodiment, the drive circuits comprise means for driving rings of light sources of different radii to electrically vary angle of illumination. These features provide excellent versatility and simple control.
In one embodiment, the circuit board comprises a plurality of separate circuits. In another embodiment, each circuit drives light sources mounted in a pattern whereby failure of a circuit has a uniform impact across the target plane. These features provides excellent fault-tolerance.
In one embodiment, the light sources are covered by epoxy, preferably applied in a continuous layer.
The epoxy layer may contain phosphorescent material.
In one embodiment, the optical axis of the lens cross-section is inclined at an angle to the normal to the plane of the annulus.
In one embodiment, the lens is inclined at 90xc2x0 to the normal to the plane of the annulus.
In one embodiment, the lens is of a deformable material.
In one embodiment, the lens has a flat arc-shaped starting configuration before being bent into an overall ring configuration in plan.
According to another aspect, the invention provides a method of producing an illuminator comprising a plurality of light sources mounted in a ring configuration and an annular lens for directing light from the light sources onto a target plane, wherein,
the annular lens is chosen to have properties such that the lens mixes light from different light sources in the tangential direction and controls spread in the radial direction so that a target plane at a desired working distance is uniformly illuminated, and
the separation of the lens from the light sources, and the light source ring diameter: lens diameter ratio are chosen to set the desired illumination area.
In one embodiment, the light source ring diameter: lens diameter ratio is chosen within a range of 1.02 to 1.10.